There are some disquieting symptoms, however, which I
don't know whether to attribute to SANE, sg, the SCSI card,
or the scanner, and on which perhaps someone can shed some light.

Symptoms:

in all configurations, interrupts seem to be disabled, or the system
seems to hang, for inordinate periods (several seconds) during a scan,
especially when the scan is first initiated, but also during each scan
segment.

At least some configurations of the system can be made to crash
by trying other activity during a scan. I have been reluctant to gather
too many statistics about just what activies cause system crashes however,
but small scans seem typically to complete OK.

Running xtops during a scan shows cpu usage 99%idle after starting
xscanimage, but before a scan is initiated, at 75% system, 25% user
during the scan, and continuing at 75/25 even after the scan has completed,
only returning to 99% idle when xscanimage is exited.

Is this behavior normal for SANE, or this card or scanner, or do I just
have something configured wrong.

The scanner is the only device on the SCSI bus, which is (at least
nominally) properly terminated, and which has worked fine for DISKS and
CDROMS.

The aha152x driver for the SCSI card is slightly ersatz on 2.2.10,
however, since that version doesn't recognize the 1520B/1522B, I added
the recognition entry from the 2.0.37 driver to the 2.2.10 driver, which
seems to make it work fine otherwise.

The card is configured with aha152x=0x340,11,7,1 in LILO, with sg, etc. as
modules. Behavior was similar with aha152x as a module.

I think this card has FIFO polled data transfer, which if properly implemented
should avoid the symptoms noted above, so I suspect the driver/card interface.